The present invention relates generally to an optical delay line apparatus and, more particularly, to a partioned optical delay line apparatus for time steering of large one-dimensional array antennas.
The state of the art of optical delay line apparatus is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents:
U.S. Pat. No. 3,368,202 issued to Crousel on Feb. 6, 1968; PA1 U.S. Pat. No. 4,671,604 issued to Soref on June 9, 1987; PA1 U.S. Pat. No. 4,671,605 issued to Soref on June 9, 1987; PA1 U.S. Pat. No. 4,714,314 issued to Yang et al on Dec. 22, 1987; PA1 U.S. Pat. No. 4,725,844 issued to Goodwin et al on Feb. 16, 1988; PA1 U.S. Pat. No. 4,814,773 issued to Wechsberg et al on Mar. 21, 1989; PA1 U.S. Pat. No. 4,814,774 issued to Herczfeld on Mar. 21, 1989; and PA1 U.S. Pat. No. 4,832,433 issued to de La Chapelle et al. on May 23, 1989.
The Crousel patent is directed to a memory core matrix and simple delay means like magnetic drum apparatus or sonic delay lines in place of the complicated interconnection network, summation equipment and shift registers which are employed in a multibeam receiving system.
The Soref patent (604) describes a wavelength dependent, tunable, optical time delay system for electrical signals having a conversion/tuning unit for converting an incoming electrical signal into an optical signal as well as selectively varying the wavelength of the optical signal. By selectively varying the wavelength of the optical signal, the electrical signal can effectively and rapidly time delayed as desired in response to the electronic signal.
The Soref patent (605) is directed to a length dependent, optical time delay/filter device for electrical signals made up of a plurality of optical fibers of varying lengths. Depending upon which fibers an optical signal (converted from an incoming electrical signal) passes through determines the time of travel of the optical signal through the device, and as a result thereof is time delayed.
The Yang et al patent discloses a mode dependent, optical time delay system for electrical signals having a highly multi-mode optical fiber having a step index profile in optical alignment with an optical source which is capable of converting an incoming electrical signal into an optical signal.
The Goodwin et al patent discusses a technique for applying selected phase delays to an optical carrier signal, the phase delays being referenced to a radio-frequency (rf) subcarrier signal. The optical signal to be phase delayed is introduced into a phase delay network comprising multiple optical paths and multiple electro-optical switches, controllable by signals generated in switching logic. The selected delays can be introduced for purposes of data modulation, or for steering an antenna beam in a phased-array antenna.
The Wechsberg et al patent describes a fiber optic feed network for a radar which couples the antenna with the transmitting and receiving circuitry. The feed system includes a set of optical multiplexers interconnected by sets of optical fibers. Microwave energy of the radar is converted to optical radiation for communication to the antenna, and then converted back to the microwave energy.
The Herczfeld patent discloses an optically controlled phased array antenna system and method of operating same utilizing fiber optic transmission lengths and controlled piezo-electric crystals or equivalent elements to introduce predetermined time delays into each light signal by controlling the respective length of each fiber optic link. The light carrying fibers are wrapped around the respective crystals in accordance with a pattern to introduce time delays corresponding to the amount of stretch given to the fiber by the energized crystals. Beam scanning is achieved by controlling the matrix of crystals to introduce appropriate time delays into the optical signals which drive the respective antenna elements.
The de La Chapelle et al patent is directed to a fiber-optic feed network using series/parallel connected light emitting optic-electronic components, such as laser diodes for distributing RF, microwave, MMW, digital signals, and pulse modulated light. The diodes are selected in number and impedance to provide a good wideband impedance match to the RF/microwave/MMW/digital driving source.
In a K-element 1-D array antenna with element to element spacing of .lambda./2, the time delay T.sub.k required by the kth element for antenna pointing at angle .theta. with respect to boresight is given by EQU T.sub.k =k.lambda.sin.theta./2c (1.1)
The total number R of different delays required for steering the antenna over a total angle .theta..sub.M with resolution .theta..sub.o, is equal to EQU R=.theta..sub.M /.theta..sub.o ( 1.2)
Typical R values are of the order of several thousands. An efficient fiberoptic delay line architecture that can easily handle this large number of delays and thus addressing each element, is the binary programmable fiberoptic delay line (BIFODEL). In the BIFODEL the microwave signal to be delayed, is converted to an optical signal via the use of a laser diode and is then routed through M fiber segments, where M=log.sub.2 (R). Each fiber segment has a length that is equal to twice the length of its right neighbor. The length of the smallest segment is such that the delay introduced corresponds to the desired delay resolution T.sub.min. Selection of the fiber segments, through which the signal is routed, can be achieved via the use of M 2.times.2 optical switches, each of which allows the signal to enter or to bypass a specific fiber segment. After the signal has been routed through the proper fiber segments, it is detected and subsequently buffered and further processed. The importance of the BIFODEL comes because the total number of delays that can be generated is equal to 2.sup.M, and thus with a small number of fiber segments a very large number of delays can be generated, e.g., for M=12 we have up to 4,096 possible delays. A BIFODEL prototype with M=8 and passband in the 500-1000 MHz band has been demonstrated in the R&D Center.
Equation 1 shows that the delay and the delay resolution T.sub.min of the BIFODEL are different for each element, and this implies that a different BIFODEL is required for each array element. This situation is impractical because it results in a large amount of hardware since current and future 1- and 2-D antenna arrays with several thousand elements are rather typical, which implies that several thousand different BIFODELs would be required. Note that a far worse situation would be faced with 2-D antennas as well, with the hardware complexity requirement being proportional to K.sup.2, where K.times.K are the array dimensions.
Efficient delay line architectures that can handle the above problems without requiring K different BIFODELs do not exist, at least in the open literature. In this disclosure there is presented a new, very powerful technique that is appropriate for large array antennas (&gt;1,000), and when used in conjunction with either of the disclosed architectures or even simple BIFODELs it realizes very large hardware savings (&gt;92 percent).
While the above-cited references are instructive, there still remains a need to provide a partitioned optical delay line apparatus for time steering of very large one-dimension array antennas. The present invention is intended to satisfy that need.